The present invention relates to the art of acoustic resonators. More particularly, the present invention relates to electro-mechanical resonators that may be used as filters for electronic circuits.
The need to reduce the cost and size of electronic equipment has led to a continuing need for smaller signal filtering elements. Thin Film Bulk Acoustic Resonators (FBARs) and Stacked Thin Film Bulk Wave Acoustic Resonators and Filters (SBARs) represent one class of filter element with potential for meeting these needs. For brevity, these filter elements will be referred to as FBARs.
An FBAR is constructed from acoustic resonators using bulk longitudinal acoustic waves in thin film piezoelectric (PZ) material. In one simple configuration, as illustrated in FIG. 1, the FBAR 100 includes a layer of PZ material 102 is sandwiched between two metal electrodes 104 and 106. The sandwich structure 100 is preferably suspended in air by supporting it around the perimeter. When an electric field is created between the two electrodes 104 and 106 via an impressed voltage, the PZ material 102 converts some of the electrical energy into mechanical energy in the form of waves. The waves propagate in the same direction as the electric field and reflect off of the electrode/air interface at some frequency including at a resonance frequency. At the resonance frequency, the device 100 can be used as an electronic resonator; hence, the device can act as a filter. Using this technique, resonators for applications in the GHz range may be constructed with physical dimensions less than 100 microns in diameter and few microns in thickness.
The FBARs can be fabricated using deposition techniques commonly used for fabricating integrated circuit elements on a substrate material. However, the FBAR fabrication processes pose unique challenges because the FBARs are preferably suspended in air. To fabricate a suspended FBAR, one technique has been to first deposit the FBAR onto a substrate, then to completely remove the substrate under the FBAR. This is illustrated in FIG. 1 where the substrate 110 under the FBAR has been removed to suspend the FBAR. However, the removal of the substrate under the FBAR exposes the FBAR and causes mechanical integrity problems. Further, it is difficult to etch the underside of the substrate.
Another technique to fabricate a suspended FBAR has been to first deposit and pattern a layer of temporary support film on the top surface of the substrate. Next, to fabricate the FBAR above the temporary support film. Then, to remove temporary support film using an undercutting etch. This technique, similar to the first technique (the substrate removal technique), causes mechanical integrity problems with the resulting FBAR. Moreover, complete removal of the temporary support film is difficult leading to inconsistent or incomplete etching of the temporary support film. And, the undercutting etch may leave effluent which may not be completely removed, causing additional problems.
Accordingly, there remains a need for an improved technique to fabricate suspended FBARs including efficient removal of sacrificial material, and for an apparatus allowing the efficient removal of the sacrificial material.
These needs are met by the present invention. According to one aspect of the present invention, an apparatus has a substrate having a depression on a top surface. On the substrate, an acoustic resonator is fabricated spanning the depression, the acoustic resonator having an etch hole providing access to the depression.
According to a second aspect of the present invention, a method of fabricating an acoustic resonator on a substrate having a top surface is disclosed. First, a depression is etched on the top surface and the depression is filled with sacrificial material. Next, the acoustic resonator is fabricated on the substrate, the acoustic resonator having an etch hole. Finally, the sacrificial material is removed.
According to a third aspect of the present invention, an apparatus has a substrate having a depression on a top surface, the depression having etch channels. An acoustic resonator is fabricated on the substrate and over the depression.
According to a fourth aspect of the present invention, a method of fabricating an acoustic resonator on a substrate having a top surface is disclosed. First, a depression is etched on the top surface of the substrate, the depression having at least one etch channel, and the depression is filled with sacrificial material. Next, the acoustic resonator on the substrate is fabricated on the substrate. Finally, sacrificial material is removed.
Other aspects and advantages of the present invention will become apparent from the following detailed description, taken in combination with the accompanying drawings, illustrating by way of example the principles of the invention.